A conventional white LED production method includes dispensing a slurry obtained by mixing a combination of R, G, B phosphors, at least one of YAG and TAG, a phosphor such as a silica-based phosphor, and R/G phosphors for increasing color rendering properties and a binder such as a two-pack type thermosetting silicone, to a near ultraviolet light-emitting diode or blue light-emitting diode using a dispenser. Another white LED production method includes coating LEDs by adding a solvent or the like to the above slurry to reduce the viscosity and directly spraying the slurry onto the LEDs using a sprayer, which is one of fine-particle generators, or the like.
Patent Literature 1 proposed by the present applicant discloses an LED production method including coating heated LED chips with a slurry that includes a phosphor, a binder, and a solvent and that is atomized by a compressed air, using a pulsed spray stream so as to form multilayer thin films while causing the slurry to adhere to also the sidewalls of the LEDs, which are believed to be difficult to coat using a typical spray method.
Patent Literature 2 proposes a layer forming method including coating LED chips with a binder such as silicone, curing the binder, coating the coated LED chips with a slurry containing a phosphor, a binder, and a solvent, and optionally mixing a dispersing agent or the like with these components.
A method using a dispenser as disclosed in Non-Patent Literature 1 and the like includes filling the above slurry to bullet LEDs, which do not produce high power, or chips inserted into the inside of a cup, such as backlight LEDs. This method is often used for mass production.
The method of Patent Literature 1 can apply the slurry while making pulsed impacts and therefore can also coat the sidewalls of the LED chips. Also, the method applies the slurry so as to form multilayer thin films and therefore can adequately cover the edges of the LED chips and significantly increase the coating efficiency. However, the method applies the slurry even to areas between LED chips incorporated into an LED substrate formed of ceramic or the like, or coats areas which need not be coated while masking the areas, thereby reducing the use efficiency of the phosphor. Particularly, in the case of a substrate in which LED chips are distant from each other, the use efficiency of the phosphor is extremely low.
The method disclosed in Patent Literature 2 includes coating the LED chips with the binder, curing the binder, and then applying the slurry containing the phosphor to the binder using an air sprayer. However, it is knowledge common to the industry that this method using a common air sprayer results in an extremely poor coating efficiency.
On the other hand, a method involving applying a slurry containing a solvent-free binder formed of silicone or the like and a phosphor using a simple device, such as a dispenser, as disclosed in Non-Patent Literature 1 and the like does not require masking and achieves a high productivity. However, as shown in FIG. 7, the center of an LED chip is swelled, and the edges thereof are thinned, resulting in a poor vertical light color temperature distribution, as well as a poor spatial color temperature distribution. Such an LED is not suitable for a high-power illumination LED. Even if a slurry is filled, for example, using reflectors in place of dams, a thick film is formed. Accordingly, a light loss occurs due to diffuse reflection or the like, resulting in a loss of the phosphor efficiency.